This is like jpcw's SD Card Holder, but on my TOM/Mk7 I needed the slots a little looser. So this looks a lot like his, but is parameterizable.
There are several parameters. The main one is for the number of slots. You can also adjust the size of the slots, if you want a tighter or looser fit. (You can also adjust other aspects, like the angle that the top rises from front to back or the car separation distance, if you're a control freak.)
You can print the 6 slot stl file that's here, or run scad and change the number of slots.